Camshafts of a roller-follower type for engines such as those used in automobiles must be able to withstand high cyclical (i.e. Hertzian) stresses with little wear. Until the advent of the present invention, only roller-follower camshafts made from steel could successfully be used for high Hertzian stress applications.
Austempered cast iron materials of high strength and high resistance to abrasion are known. For example, U.S. Pat. No. 3,549,431 to De Castelet, issued Dec. 22, 1970; U.S. Pat. No. 3,860,457 to Vuorinen et al., issued Jan. 14, 1975; U.S. Pat. No. 4,541,878 to Muehlberger et al., issued Sept. 17, 1985; U.S. Pat. No. 3,893,873 to Hanai et al., issued July 8, 1975; U.S. Pat. No. 3,549,430 to Kies, issued Dec. 22, 1970; U.S. Pat. No. 2,485,760 to Millis et al., issued Oct. 25, 1949; U.S. Pat. No. 2,324,322 to Reese et al., issued July 13, 1943; and U.S. Pat. No. 3,273,998 to Knoth, et al., issued Sept. 20, 1966, disclose austempered cast iron compositions. However, each of the processes disclosed fails to yield a form of cast iron which has a hardness suitable for machine elements in rolling contact such as roller-follower camshafts and which is prepared in a time-efficient manner to reduce overall manufacturing costs. Nor do these prior patents disclose an efficient means by which it is possible to selectively austemper portions of an article, thereby reducing overall costs and manufacturing time.
Grindahl discloses a cast iron article in the form of a gear that provides high resistance to wear. However, the Grindahl process includes the step of holding the article at an austenitizing temperature for a time preferably in the range of 3.5 hours. Grindahl's article also undergoes a cold-working step as part of the process.
De Castelet discloses a cast iron which is austempered at a temperature that yields a hardness too low for articles so made to resist wear when in rolling contact. In addition, although De Castelet discloses that articles may have portions thereof heat-treated, he does not disclose an efficient means to accomplish such localized heat treatment.